1. Field of the Invention:
The present invention relates to well pump apparatus, and in particular to an anchorable pack-off assembly and a method for seating the assembly for use within the tubing of a well, in particular an oil well, and to well pump apparatus in which the effects of fluid pound are countered.
2. Description of the Prior Art
My U.S. Pat. No. 3,945,774, which is fully incorporated herein by this reference, discloses an anchorable pack-off assembly for use in the well tubing of an oil well, in which the tubing constitutes an outer pump casing (pump barrel). The pack-off assembly is included in a structure in which first and second one-way valves are provided in the tubing at opposite ends of the pump structure. A hollow member having an outer diameter less than the inner diameter of the tubing is disposed within the tubing and connected to a string of sucker rods for reciprocation. The hollow member is sealed to the inner surface of the tubing by a pack-off assembly which divides the volume between the upper and lower plungers into separate pumping chambers. The hollow member includes fluid passages above the pack-off assembly to provide fluid communication with the upper chamber so that during an up stroke fluid is drawn through the first valve into the lower chamber and, during a down stroke, fluid is expelled by way of the second valve carried on the upper plunger, from the lower chamber through the hollow member and from the upper chamber via the fluid passages and upward through the second valve and into the tubing above the second valve.
During installation, the pack-off assembly is pushed into the tubing by the upper plunger and sealed thereto by the way of annular friction seating rings on the body seating member of the body of the anchorable pack-off assembly. At times, it was found, that there was a difficulty in installation. The sizing of the friction seating rings on the body of the anchorable pack-off assembly is particularly critical. If these rings are a few thousandths of an inch too small, the pressure of the upper plunger pumping thereagainst during operation of the pump would displace the assembly. If the friction seating rings are a few thousandths of an inch too large, the assembly cannot be forced into the barrel. This problem is compounded by the American Petroleum Institute's allowable tolerance of the sizing of working barrels, which is the nominal size, +0.005", thus making a tolerance range of 0.010".